The present disclosure relates to suppression of noise in marine electromagnetic surveying and, more particularly, to a method and system for suppression of noise that includes combining the estimation of electromagnetic earth responses and ambient noise into a technique that makes use of models for both contributions.
One type of marine geophysical surveying includes marine electromagnetic surveying, in which geophysical data may be collected or acquired from below the Earth's surface. Marine electromagnetic surveying is used, among other purposes, to infer spatial distribution of electrical conductivity of formations below the bottom of a body of water, such as a lake or ocean. The spatial distribution of conductivity is used to assist in determining the presence of hydrocarbon-bearing formations in the subsurface, potentially resulting in cost saving by better targeted drilling operations. Marine electromagnetic surveying generally includes inducing an electromagnetic field in the subsurface formations and measuring one or more parameters related to a response of the subsurface formations to the induced electromagnetic field. The current may diffuse through the sea water and subsurface formation with an electric potential difference caused by the current measured at some distance away from the electromagnetic energy source. The sensitivity of the potential difference to the sub-surface resistivity structure may depend on the sensor offsets and the source signal frequencies. For example, electromagnetic sensors may be distributed on one or more streamers towed by a survey vessel, located on one or more ocean bottom cables, or fixed at nodes at or near at a water bottom. The ability to invert for the sub-surface resistivity may be limited by a combination of sensitivity to resistivity changes and the signal-to-noise ratio.
Accordingly, there is a need for improved methods for suppressing noise in marine electromagnetic surveying, for example, to improve data quality and enlarge the weather window for marine electromagnetic surveys. An embodiment that may be used to find the best possible or practicable resistivity map of the subsurface to improve hydrocarbon exploration may use an operator, or method, that finds resistivity parameters as close to the true ones as possible or practicable with minimal error. The more information used, such as seismic data, well logs, geological information, etc., at the beginning of the method may allow for an improved estimate of the resistivity parameters. Noise characteristics may be used to further improve the resistivity estimate.